Centrifuge separators are frequently used to separate mixtures containing fluids of different densities. In operation, centrifuges generally involve feeding the mixture to be separated into a cylindrical rotor capable of being rotated about its central axis at high speed. Centrifugal force causes the components to collect in layers along the inner wall of the rotor. The layers are then individually removed from the rotor.
A number of different parameters are used to characterize the operational efficiency of centrifugal separators. One such parameter is the volume of the input mixture which can be treated in a given time period. For example, oilfield separation volumes are stated as a number of barrels per day of the input mixture which can be separated.
Other parameters characterize efficiency in terms of the quality of the separated fluids. These parameters are stated as a percentage, or as a number of parts per million, of impurities in each separated fluid. In oilfield separation, where the mixture to be separated contains crude oil and water, the typically computed impurity parameters are the amount of water remaining in the separated oil, and the amount of oil remaining in the separated water. Of these two parameters, the oil remaining in the separated water is typically the parameter whose target value is more difficult to attain. Most common centrifuge separators attain satisfactory water in oil impurity levels.
The challenge in the centrifuge art is to develop separators which maximize the volume of a mixture treated in a given time period, while simultaneously minimizing the impurities in the output fluids. This challenge is particularly acute in the oilfield production setting, where high daily volumes must often be separated. For example, oilfield production separators often must separate several thousand barrels of liquids per day. Despite these high volumes, regulatory, environmental, and refinery constraints all generally require the separated fluids to have minimum impurities. A typically quoted requirement for the amount of oil in water is 40 parts per million.
The problem that the centrifuge designer faces is that the maximum volume and minimum impurity goals to some extent involve conflicting technical considerations. For example, increasing the throughput volume on typical centrifuge designs is not always possible, and, where possible, may create undesirable flow characteristics within the centrifuge rotor. These result from the fact that the input mixture must be quickly accelerated to the speed of the rotor. The flow characteristics may include unsteady or turbulent flow regimes, vortex shedding, mixing or shear flow zones, fluid interfacial instabilities, and the like. None of these impact throughput volume, but they all may impact output fluid quality. More specifically, it is generally understood in the centrifuge art that any flow process that tends to increase fluid mixing or turbulence, or cause dissimilar motions between the particles of the fluids to be separated, increases the level of impurity in the output fluids. Therefore, existing centrifuges generally involve a tradeoff between throughput volume and separation efficiency.
An additional complication that sometimes faces separation equipment used in oilfield production is dissolved matter in the input mixture. Production mixtures may contain wax and other matter, which, as a result of the generally high temperature of the mixture, is in solution form. As the separation process occurs, however, that wax may form deposits on internal portions of the separator, reducing both volume and impurity efficiency. Oilfield centrifuges may also be subject to the internal accumulation of sand or solids which also reduce separation efficiency.
U.S. Pat. No. 4,846,780 to Galloway et al. ("Galloway") is an example of a prior art centrifuge separator. In its principal embodiment, Galloway uses a liner along the inner wall of the rotor to create a two pass separation process. The liner creates a complex passageway in which wax and other matter may gather, ultimately reducing separator s efficiency. Input to the Galloway centrifuge is via a nozzle which sprays fluids into an impeller for acceleration out to the inner wall of the rotor. Flow out of the nozzle is not tightly controlled, however, and is a highly turbulent process. The Galloway centrifuge can be fabricated as a one pass separator without the complex passageway, but the level of impurity of the separated fluids is increased accordingly.
From the foregoing, it can be seen that a centrifuge separator is needed that does not sacrifice separation is efficiency for throughput volume, that does not involve complex fluid flow patterns, that minimizes fluid mixing and turbulence during the separation process, and that involves simplified internal passageways which promote cleaning and minimize the deposition of solid matter. The present invention satisfies that need.